Donor-Acceptor Polymeric Materials: Syntheses, Properties, and Their Memory Device Applications
Date Issued
2011
Date
2011
Author(s)
Fang, Yi-Kai
Abstract
Donor-acceptor type polymers have attracted a significant interest for memory device applications due to their tunable electronic properties through molecular design. However, the effects of the different polymer structure on the memory characteristics have not yet been explored. In this thesis, we address the above issues by exploring the following subjects: (1) design and synthesis of new donor-acceptor organic polymers, (2) preparation and characterization of the memory devices, and (3) correlation of the donor-acceptor polymer structure or composition with memory device characteristics.
In the first part of this thesis (chapter 2), the synthesis, structures, morphology, optoelectronic and the memory device properties of poly(3-hexylthiophene)-block-poly(vinylphenyl oxadiazole) donor-acceptor rod-coil block copolymers are reported. The novel donor-acceptor rod-coil diblock copolymers of regioregular poly(3-hexylthiophene) (P3HT)-block-poly(2-phenyl-5-(4-vinylphenyl) -1,3,4-oxadiaz-ole) (POXD) were successfully synthesized contain three different the coil lengths of 5, 18 and 25 repeating units by the combination of a modified Grignard metathesis reaction (GRIM) and atom transfer radical polymerization (ATRP). The small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) demonstrate that the decreased crystallinity of the block copolymers were duo to the increasing POXD block ratio was depress the crystallization of P3HT. The DSC, UV-vis, CV, TEM and AFM are similar to the SWAXS results. Homopolymers and block copolymers were synthesized and used to evaluate the memory applications. Mismatch of the metal work function relative to the low-lying HOMO and high-lying LUMO levels of POXD, and served as an insulator and trapping center. These block copolymers depend on different the coil (POXD) lengths exhibited different characteristics of these memory devices. The P3HT44-b-OXD5 and P3HT33-b-OXD25 memory devices are hole-transport diode and insulator, respectively. The P3HT44-b-OXD18 memory devices exhibited non-volatile with NDR behavior.
In the second part of this thesis (chapter 3), a series of non-conjugated random copolymers containing pendent electron-donating 9-(4-vinylhenyl)carbazole (VPK) and electron-withdrawing 2-phenyl-5-(4-vinylphenyl)1,3,4-oxadiazole (OXD) or 2-(4-vinylbiphenyl)-5-(4-phenyl)- 1,3,4-oxadiazole (BOXD) with three different ratio (8/2, 5/5, 2/8) were successfully synthesized by nitroxide-mediated free radical polymerization (NMRP) method. The prepared random copolymers are denoted as P(VPKxOXDy) or P(VPKxBOXDy) with three different electron donor/acceptor (x/y) ratios of 8/2, 5/5, and 2/8. The electrical switching behavior based on the ITO/polymer/Al device configuration could be tuned through the donor/acceptor ratio or acceptor trapping ability. Both experimental and theoretical results indicated that the charge transfer between the pendant donor and acceptor was relatively weak without significant orbital hybridization. In addition, the low-lying HOMO energy level of OXD or BOXD units was employed as charge trapping site. Therefore, distinct electrical current-voltage (I-V) characteristics changed from diode, to volatile memory, and insulator depending on the relative donor/acceptor ratios of 10/0, 8/2, and (5/5, 2/8 and 0/10), respectively. The memory device based on P(VPK8OXD2) or P(VPK8BOXD2) copolymers exhibited volatile static random access memory (SRAM) behavior with an ON/OFF current ratio of approximately 104-105, up to 107 read pulses, and retention time of more than 1 h. The unstable ON state in the device was due to the shallow trapped holes with spontaneously back transferring of charge carriers when the electric field was removed and thus exhibited the volatile nature. The slightly lower HOMO level of OXD moieties than that of BOXD led to P(VPK8OXD2) device storing the charge for a longer period of time. The present study suggested the high performance polymer memory devices could be achieved by changing the donor/acceptor ratio or chemical structure.
In the third part of this thesis (chapter 4), we report the synthesis and resistive-type switching memory characteristics based on new random copolymers of P(VTPAxBOXDy) and P(CNVTPAxBOXDy) containing different donor/acceptor ratios (8/2, 5/5, and 2/8) of pendent electron-donating 4-vinyltriphenylamine (VTPA) or 4,4′-dicyano-4″-vinyl-triphenylamine (CNVTPA) and electron-withdrawing 2-(4-vinylbiphenyl)-5-(4-phenyl)-1,3,4-oxadiazole (BOXD). The effects of donor/acceptor ratios and cyano side group on the memory characteristics were explored and compared with preperties of homopolymers, PVTPA, PCNVTPA, and POXD. The distinct electrical current-voltage (I-V) characteristics of the ITO/P(VTPAxBOXDy) /Al device changed from volatile memory to insulator depending on the relative donor/acceptor ratios. The ITO/P(VTPA8BOXD2) or PVTPA/Al device exhibited DRAM and SRAM behavior, respectively, with an ON/OFF current ratio of 107-108. However, no switching phenomena were observed for a higher BOXD ratio. Moreover, the devices could endure 108 cycles under a voltage pulse and show a long retention time for at least 104 s under a constant voltage stress. The low-lying HOMO energy level of BOXD was employed as hole-blocking units as the charge transport were occurred between the neighboring triphenylamine units. The charge trapping/spontaneously back transferring of trapped carriers controlled the switching behavior. On the other hand, all the P(CNVTPAxBOXDy) memory devices exhibited non-volatile nature with NDR behavior due to the preferred interaction of Al atoms with the cyano group. Here, the results demonstrated that the pendent polymers with specific donor-acceptor chromophores could tune the memory switching characteristics for advanced electronic device applications.
Subjects
Donor-Acceptor
Pendent
Memory
Morphology
Type
thesis
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